Many polymer materials are known. Numerous polymers have found use in lubricating oils as viscosity improving agents. These materials are also often referred to as viscosity index improvers. Multifunctional additives that provide both viscosity improving properties and dispersant properties are likewise known in the art. Such products are described in numerous publications including Dieter Klamann, "Lubricants and Related Products", Verlag Chemie Gmbh (1984), pp 185-193; C. V. Smalheer and R. K. Smith, "Lubricant Additives", Lezius-Hiles Co (1967); M. W. Ranney, "Lubricant Additives", Noyes Data Corp. (1973), pp 92-145, M. W. Ranney, "Lubricant Additives, Recent Developments", Noyes Data Corp (1978), pp 139-164; and M. W. Ranney, "Synthetic Oils and Additives for Lubricants", Noyes Data Corp. (1980), pp 96-166. Each of these publications is hereby expressly incorporated by reference.
The viscosity of lubricating oils, particularly the viscosity of mineral oil based lubricating oils, is generally dependent upon temperature. As the temperature of the oil is increased, the viscosity usually decreases. The function of a viscosity improver is to increase the kinematic viscosity of an oil at elevated temperatures with minimal increases in viscosity at low temperature. Thus, a viscosity improver enables an oil containing it to resist significant changes in viscosity with changes in temperature.
Dispersants are also well known in the lubricating art. Dispersants are employed in lubricants to keep impurities, particularly those formed during operation of an internal combustion engine, in suspension rather than allowing them to deposit as sludge.
One type of compound having both viscosity improving and dispersancy properties is comprised of a polymer backbone onto which backbone has been attached one or more monomers having polar groups. Such compounds are frequently prepared by a grafting operation wherein the backbone polymer is reacted directly with a suitable monomer.
Several methods for preparing such grafted polymers are known. One method involves thermal grafting of an activated olefin monomer onto a backbone containing unsaturated carbon-carbon bonds by a process known as the "ene" reaction. For example, maleic anhydride can be grafted onto an ethylene propylene diene modified (EPDM) polymer backbone. Usually the succinic derivative so obtained is further reacted with polar group containing reagents, such as amines, alcohols, etc. to provide dispersant-viscosity improvers. See for example U.S. Pat. No. 4,320,019 and U.S. Pat. No.4,357,250.
Another method for preparing graft copolymers for use as dispersant-viscosity improvers involves reacting a hydrocarbon based polymer backbone with a monomer containing a polar group in the presence of a free radical initiator. Numerous patents deal with the subject including U.S. Pat. No. 3,089,832; U.S. Pat. No. 4,181,618; and U.S. Pat. No.4,358,565.
The free-radical graft process is, in principle, a simple way to provide a measured degree of polarity to a hydrocarbon backbone polymer. In practice, however, several problems are encountered. Often the free-radical initiator will cause significant cross-linking of the polymer backbone, resulting in poor oil solubility and reduced effectiveness as a viscosity improver.
Furthermore, the use of alkyl peroxide initiators at high temperatures (in the range of 120-180.degree. C.) frequently results in degradation of the polymer backbone, diminishing the average molecular weight and resulting in an undesirable loss of thickening power in the final product.
Other problems arise if the polar vinyl monomer itself undergoes significant homopolymerization or oligimerization rather than participating in the grafting process. As mentioned above, the purpose of the free radical graft process is to impart a measured degree of polarity to a hydrocarbon polymer when it is successfully attached to backbone. Ideally, the graft monomer will attach to the backbone polymer in monomeric units. Homopolymerization of the graft monomer is detrimental in several ways, and can adversely affect the nature of the products:
1. The polar homopolymer so produced is usually insoluble, and the resulting product is nonhomogeneous and hazy in appearance. Filtration of a highly viscous polymer solution to try to improve clarity by removing the suspended homopolymer is an undesirable processing step since it presents handling difficulties, increased time cycles and relatively poor efficiency.
2. The homopolymers so introduced are frequently detrimental to performance in lubricants, particularly when used at high temperatures or under oxidizing conditions.
3. Monomers converted to homopolymer are not available for grafting onto the resin substrate, and the effect of the polar group in providing the desired dispersancy is lost.
4. The grafting process becomes inefficient with respect to the utilization of raw materials.
The grafting process is often carried out in the presence of a solvent. The solvents employed in the prior art have generally been the same solvents used for preparing the backbone polymer. That is, they are present to reduce viscosity and to facilitate processing. These solvents are often specified as saturated hydrocarbons (e.g., cyclohexane) or haloaromatic compounds, neither of which normally undergo hydrogen-atom transfer to any significant degree. See, for example, U.S. Pat. No. 4,358,565, which teaches that the grafting process, carried out in the presence of free radical polymerization initiator, is suitably carried out in solvents which have a very low reactivity towards free radicals, e.g., dichlorobenzene, benzene and preferably, cyclohexane.
The problem of gelling has been recognized. European Patent Application 171167 teaches that gelling can be reduced by conducting the reaction in the presence of a free radical initiator and a chain stopping agent. Aliphatic mercaptans are preferred. Tertiary mercaptans and N,N-diethyl hydroxylamine are most preferred. Other chain stopping agents disclosed are cumene, alcohols, phenols, etc. This European Patent Application deals with chain stopping agents as a general class and prefers certain reactive materials as chain stopping agents. As will be discussed hereinafter, reactive chain stopping agents are not desirable when the graft polymer, prepared in the presence of these agents, is intended for use in lubricating oil compositions, and because they may interact in an adverse manner with the vinyl-nitrogen monomer.